There are many applications for stationary or slow-moving platforms located from a few tens to thousands of feet above the ground, including overhead photography, video observation, and relaying communications. Some applications may require a platform which can remain stationary for many hours, for example for security monitoring. Others may require a platform which can be quickly deployed, for example for police or emergency services, or news gathering.
Existing options for such platforms include fixed or deployable (e.g., truck-mounted) towers or masts, manned helicopters, tethered or free-flying lighter-than-air (LTA) vehicles such as blimps, and vertical-takeoff-and-landing (VTOL) unmanned aerial vehicles (UAVs) using either battery power or fuel-burning engines. These have various disadvantages. For example, towers and masts in non-permanent applications are limited to a few tens of feet of height. Manned helicopters are expensive to purchase and operate, noisy, and have limited flight time. Existing UAVs, particularly VTOL UAVs, also have limited flight time, and are not allowed to fly in many areas. Fuel-burning VTOL UAVs are noisy. Tethered blimps are difficult to deploy, require expensive helium, and are not usable in high winds.
There exist VTOL UAVs which are powered by electricity transmitted through a tether containing electrical conductors, for example the IAI (Israeli Aircraft Industries) ETOP (Electric Tethered Observation Platform). However, electrical tethers are necessarily heavy, either due to the weight of the metallic conductors or, if high voltage is used to reduce the conductor size, the weight of the insulation. Electrically-conducting tethers also present a safety hazard in the presence of lightning or overhead power lines.
It is known in the art to provide power for propulsion to an airborne platform via free-space transmission of electromagnetic radiation, at either microwave or optical wavelengths. Free-space transmission is the transmission of energy from one location to another location without the use of a conduit that couples the two locations to each other, for example a wire or a cable. However, free-space transmission presents hazards associated with exposure to high power microwaves or lasers, and requires regulatory approvals and stringent safety measures. Free-space laser transmission is also subject to blockage by clouds, fog, or precipitation.
It is also known in the art to provide, through a tether which includes a tube or hose, a flammable/explosive fuel to an airborne platform that uses a combustion engine for propulsion. Providing flammable/explosive fuel through a tube leads to a heavy tether, and presents a fire or explosion hazard if the tube is damaged. In addition, combustion engines are noisy, and require frequent maintenance.
There is therefore a need for a class of platforms which can avoid many of these disadvantages, and in particular which can be deployed quickly, remain in place for long periods, fly safely and quietly, operate stably in a wide range of conditions, and readily reach altitudes up to at least several hundred feet.